1. Field of the Invention
Embodiments of the invention relate to drive circuits for driving insulated gate switching elements such as an IGBT (Insulated Gate Bipolar Transistor) or MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
2. Related Art
A semiconductor device that has packaged therein an insulated gate switching element, such as an IGBT, and a drive circuit for driving the switching element is called “IPM (Intelligent Power Module)”. A power switching element such as an IGBT is mounted on the IPM for driving a motor or the like. Application of excess current to this power switching element can immensely damage an electronic device equipped with the IPM. For this reason, the semiconductor device is provided with a self protection function that constantly monitors the current flowing to the power switching element and safely stops the control by discontinuing the supply of gate signals when an excess current beyond a predetermined current value flows to the power switching element.
Such a conventional IPM adopts an IGBT drive system that connects a P-channel MOSFET 51 and an N-channel MOSFET 52 in series, as shown in FIG. 4. The source of the P-channel MOSFET 51 is connected to a power supply voltage Vcc, while the source of the N-channel MOSFET 52 is connected to a ground potential. The drains of the P-channel MOSFTET 51 and the N-channel MOSFET 52 are connected to the gate of an IGBT 53, and a drive signal is input to the gate of the P-channel MOSFET 51 and the gate of the N-channel MOSFET 52.
When turning the IGBT 53 ON, the level of the drive signal is lowered so that the P-channel MOSFET 51 is turned ON and the N-channel MOSFET 52 is turned OFF. Consequently, the power supply voltage Vcc is applied to the gate of the IGBT 53 via the P-channel MOSFET 51.
When turning the IGBT 53 OFF, on the other hand, the level of the drive signal is increased so that the P-channel MOSFET 51 is turned OFF and the N-channel MOSFET 52 is turned ON. Consequently, the ground potential is applied to the gate of the IGBT 53 via the N-channel MOSFET 52.
In this configuration, ON-resistances of the P-channel MOSFET 51 and the N-channel MOSFET 52 are used for driving the IGBT 53 so as to turn the IGBT 53 ON and OFF.
In this system where the IGBT 53 is driven by the ON-resistances of the P-channel MOSFET 51 and the N-channel MOSFET 52, the ON-resistances of the P-channel MOSFET 51 and the N-channel MOSFET 52 increase at a temperature higher than a room temperature. Therefore, when the temperature is higher than the room temperature, the charge rate at the gate of the IGBT 53 slows down and a steep voltage change (voltage between the collector and the emitter of an IGBT 43) is prevented, alleviating the occurrence of noise related to the voltage change. Nevertheless, the problem is the increase in losses due to an increase in time required to turn the IGBT 53 ON. However, when the design of the semiconductor device is optimized in order to minimize the losses in high temperatures, the charge rate at the gate of the IGBT 53 becomes extremely low at the room temperature, causing a steep voltage change and consequently increasing noise.
In order to solve these problems, Patent Document 1 proposes a drive circuit for an insulated gate device.
In Japanese Patent Application Publication No. 2008-103895 (also referred to herein as “Patent Document 1”), the drive circuit for an IGBT is provided with a constant current source generating a constant current and configured by a current mirror circuit, a switching circuit that connects a gate of the IGBT to a power supply potential side via the constant current source when turning the IGBT ON and disconnects the gate of the IGBT from the power supply potential side via the constant current source when turning the IGBT OFF, and a discharge circuit that turns the IGBT OFF. When the level of a drive signal becomes low, the switching circuit connects the gate of the IGBT to the power supply potential side via the constant current source, to turn the IGBT ON. When, on the other hand, the level of the drive signal becomes high, the switching circuit terminates the connection made between the gate of the IGBT and the power supply potential side via the constant current source, and the discharge circuit connects the gate of the IGBT to a ground potential to turn the IGBT OFF.
Japanese Patent Application Publication No. 2009-11049 (also referred to herein as “Patent Document 2”) proposes a gate drive circuit that has a constant-current-pulse gate drive circuit that creates a gate signal for a voltage drive switching device such as an IGBT or FET as a constant-current output, a constant-voltage-pulse gate drive circuit that creates the gate signal as a constant voltage output, and a gate drive circuit that has a decision/switch circuit that switches between an operation of the constant-current-pulse gate drive circuit and an operation of the constant-voltage-pulse gate drive circuit.
Japanese Patent Application Publication No. 2005-260752 (also referred to herein as “Patent Document 3”) proposes a technology in which a current on the input side of a constant current source configured by a current mirror circuit is adjusted by bypassing one of the serially-connected resistors by means of a switch element, to change an output current of the constant current source.
Japanese Patent Application Publication No. 2000-40849 (also referred to herein as “Patent Document 4”) proposes a technology in which, as with Patent Document 3, a collector of one of PNP transistors configuring a current mirror circuit is connected to a terminal of a ground potential via a variable resistance resistor circuit, and a resistance of the variable resistance resistor circuit is selected using a resistance selecting part, to change an output current of the constant current circuit.
International Patent Publication No. WO 2008/155917 (also referred to herein as “Patent Document 5”) proposes a switching element drive circuit that drives a switching element such as an IGBT or MOSFET for switching large power. This switching element drive circuit uses a drive signal output circuit for driving the switching element, to, first, output an increased voltage V2 to a gate of the switching element when a PWM pulse that is input from a PWM pulse output circuit is at a high level, and to, subsequently, output a predetermined voltage V1 lower than the increased voltage V2 to the gate of the switching element when a gate voltage Vgs at the switching element rises to a predetermined voltage. This configuration can prevent switching losses of the switching element.
The demand for low current consumption in the IPM has been increasing, and the amount of current consumed by the IPM has a high proportion of the amount of current consumed by a drive circuit for driving an IGBT and a control circuit for protecting the IGBT.
In the conventional example described in Patent Document 1 above, the insulated gate device can be turned ON via the constant current source, and the temperature dependence of the charge rate of the gate of the insulated gate device can be reduced. Therefore, when turning the insulated gate device ON, the noise caused in a high temperature period and room temperature period can be minimized. Although being able to minimize the noise and losses, the conventional example described in Patent Document 1 has an unsolved problem of not being able to reduce the amount of current consumed by the drive circuit.
In the conventional example described in Patent Document 2, the insulated gate device can be switched ON in a constant time period by driving the insulated gate device at a constant current until the gate voltage exceeds a predetermined voltage. In addition, by switching the mode thereafter to a constant voltage drive mode, the device can be driven without undermining the credibility of the gate oxide film of the device. However, Patent Document 2 has an unsolved problem of not being able to reduce the amount of current consumed by the drive circuit.
Moreover, the conventional examples of Patent Documents 3 and 4 disclose that the current values of the constant current circuits are changed by changing the resistances, but do not at all describe drive circuits of the insulated gate switching elements.
In addition, according to the conventional example described in Patent Document 5, when turning ON the switching element such as an IGBT or MOSFET for switching large power, an increased voltage, higher than a predetermined voltage applied constantly the gate, is applied only for an initial period until the gate voltage of the switching element reaches the predetermined voltage. As a result, the switching operation can be performed promptly without constantly applying an excess voltage to the gate of the switching element when turning the switching element ON. Therefore, this conventional example can shorten the delay time and reduce the switching losses without adding extra stress to the gate of the switching element. However, these conventional examples have certain shortcomings, such as, for example, not being able to reduce the amount of current consumed in the drive circuit thereof.